The remobilization of plant resources during stress facilitates environmental adaptation. The definitive example of such metabolite reallocation in a leaf is delineated by leaf senescence. In addition to natural ageing, many environmental factors such as temperature, drought, nutrient supply, pathogen attack and light conditions can hasten senescence. For example, dark-induced stress is characterized by leaf yellowing, due to the breakdown of chlorophyll and general chloroplast degradation. Metabolic changes during senescence are associated with the transition from nutrient assimilation to metabolite turnover that is accelerated by catabolic activities. For example, in legumes, the catabolic products of purine, the ureides, can provide the plant with readily transportable metabolites that excel in a high nitrogen to carbon ratio. It has been suggested that the carbon nitrogen ratio generated in energy-conserving purine catabolism may be critical for the plants survival under stress conditions.
Little is known about the control of purine catabolism in leaves. Purine catabolism starts with the conversion of adenosine monophosphate (AMP) to inosine monophosphate (IMP) by AMP deaminase (AMPD, E.C.3.5.4.6) that leads by multiple pathways to the production of oxypurines such as xanthine and hypoxanthine. However, further degradation of xanthine to urate requires the activity of the pivotal enzyme, xanthine dehydrogenase (XDH, EC 1.1.1.204). Urate is further metabolized to the ureide, allantoin, through urate oxidase (UO) and transthyretin-like protein (TLP). In Arabidopsis, allantoinase (ALN, E.C. 3.5.2.5.) converts allantoin to allantoate, followed by allantoate amidinohydrolase (AAH, E.C. 3.5.3.9.), that converts allantoate to ureidoglycolate and ammonia. A putative ureidoglycolate lyase converts ureidoglycolate to the basic metabolic building blocks, glyoxylate and urea.
XDH contains molybdenum cofactor (MoCo), FAD and NADPH binding domains. The second oxygen in the mono-oxo-MoCo is replaced by a sulfur ligand. When using xanthine/hypoxanthine or NADH as substrates the sulfo-MoCo form of XDH can generate superoxide radicals. In contrast, the desulfo-MoCo form of XDH shows only FAD dependent activity and generats superoxide radical only in the presence of NADH.
Among the two XDH encoding genes detected in the Arabidopsis genome (AtXDH1 and AtXDH2), only AtXDH1 responds to environmental stimuli. A specific function of XDH in ageing is not clear, although in both mammalian heart and in plants the activity of XDH is enhanced with age.
Nowhere in the background art is it taught or suggested that ureide metabolism can be regulated to increase plant resistance to stress. Exogenous application of ureides or enhanced endogenous accumulation of ureides for plant protection and, for example, extension of shelf life of produce has not been previously known or suggested in the art.